A variety of electromagnetic shielding assemblies are known in the art. Such assemblies are typically constructed as either single piece assemblies or two-piece assemblies. The single piece assemblies typically have a top portion integral with a plurality of side portions that extend downwardly from the top portion. The two-piece assemblies typically have a base portion and a separate cover. The base portion provides the walls of the two-piece shielding assembly and typically includes a plurality of downward extending protrusions that are used to align the base portion during placement onto a printed circuit board. The cover typically comprises a substantially planar portion and typically includes a plurality of so-called spring fingers extending downwardly about a periphery of the cover. Both types of shielding assemblies are generally fabricated from a metallic composition, such as a tin-plated aluminum alloy.
Attachment of the single piece assemblies is typically accomplished by reflow soldering the side portions to a receptacle trace on the printed circuit board that encircles a circuit to be shielded. Solder paste (solder and flux) is placed onto the receptacle trace and circuit component solder pads. The circuit components are automatically and/or manually placed upon their corresponding pasted solder pads. The single piece shielding assembly is then manually placed onto the pasted receptacle trace. The printed circuit board is reflowed in an infra-red reflow oven such that the circuit components and the shielding assembly are electrically connected to their respective traces and pads on the circuit board.
Several problems arise with the use of such single piece shielding assemblies. First, the single piece shielding assembly requires manual placement prior to fellow. Manual placement is generally a much tower quality--and hence, a more costly--process than automated placement. Second, height tolerances of the side portions commonly inhibit complete soldering of the side portions, thereby creating unintended apertures in the solder joint between the shielding assembly and the receptacle trace. Depending on the size of the apertures, the shielding effectiveness of the shielding assembly may be significantly degraded. Finally, the single piece shielding assembly must be de-soldered in order to gain access to the shielded circuit components. Thus, repair or replacement of a shielded component requires sufficient heating to remove the single piece shielding assembly.
Attachment of the two piece shielding assemblies is typically accomplished by soldering the base portion to a receptacle trace on the printed circuit board using a post-reflow waveline soldering process and subsequently attaching the cover to the base portion. Prior to reflow, either solder paste is placed onto the receptacle trace and the circuit component solder pads, or the receptacle trace and the solder pads are pre-tinned. The circuit components are then automatically and/or manually placed upon their corresponding solder pads. The printed circuit board is reflowed in an infra-red reflow oven to electrically connect the components to their respective pads on the circuit board. The base portion is then manually placed onto the receptacle trace by aligning the protrusions in the base portion with corresponding slots in the printed circuit board. The base portion is soldered to the receptacle trace using a known waveline soldering process. After the base portion is attached, the cover is pushed onto the base portion such that the spring fingers are securely connected to the base portion.
There exists two problems with using existing two piece shielding assemblies. First, the base portion requires manual placement prior to waveline soldering. As mentioned above, manual placement is a lower quality, more costly process than is automated placement. Secondly, soldering of the base portion is a secondary process that provides an undesired increase in the cycle time for processing component-laden printed circuit boards.
Therefore, a need exists for a method and apparatus that provide electromagnetic shielding of an electrical circuit that is disposed on a substrate, while facilitating single process reflow soldering and automated placement of an effective base portion of an electromagnetic shielding assembly. Further, an electromagnetic shielding assembly that accounts for height tolerances in the side walls of single piece assemblies would be an improvement over the prior art.